1. Technical Field
This invention relates to fuel cell systems and, more particularly, to procedures for shutting down an operating fuel cell system.
2. Background Information
It is well known in the fuel cell art that, when the electrical circuit is opened and there is no longer a load across the cell, such as upon and during shut-down of the cell, the presence of air on the cathode, coupled with hydrogen fuel remaining on the anode, often cause unacceptable anode and cathode potentials, resulting in catalyst and catalyst support oxidation and corrosion and attendant cell performance degradation. It was thought that inert gas needed to be used to purge both the anode flow field and the cathode flow field immediately upon cell shut-down to passivate the anode and cathode so as to minimize or prevent such cell performance degradation. Further, the use of an inert gas purge avoided the possible occurrence of a flammable mixture of hydrogen and air, which is a safety issue. While the use of 100% inert gas as the purge gas is most common in the prior art, commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen as the anode side purge gas, and a cathode side purging mixture comprising a very small percentage of oxygen (e.g. less than 1%) with a balance of nitrogen. Both of these patents also discuss the option of connecting a dummy electrical load across the cell during the start of purge to lower the cathode potential rapidly to between the acceptable limits of 0.3-0.7 volt.
It is undesirable to use nitrogen or other inert gas as a shut-down or start-up purge gas for fuel cells where compactness and service interval of the fuel cell powerplant is important, such as for automotive applications. Additionally, it is desired to avoid the costs associated with storing and delivering inert gas to the cells. Therefore, safe, cost effective shut-down and start-up procedures are needed that do not cause significant performance degradation and do not require the use of inert gases, or any other gases not otherwise required for normal fuel cell operation.
In accordance with the present invention, a fuel cell system, which recirculates a portion of the anode flow field exhaust through the anode flow field in a recycle loop during operation, is shut-down by disconnecting the primary load from the external circuit and thereafter stopping the flow of fresh hydrogen containing fuel into the anode flow field and catalytically reacting hydrogen in the recirculating anode exhaust by recirculating such gases within the recycle loop into contact with a catalyst until substantially all the hydrogen in such gases is removed. Preferably, before the flow of fuel to the anode is stopped, upon disconnecting the primary load the oxidant flow to the cathode flow field is halted and a small auxiliary load is connected across the cell for a period of time to lower the cell voltage and cathode potential. An inert gas purge of the cell is not used or required as part of the shut-down procedure.
In one experiment using a stack of PEM fuel cells of the general type described in commonly owned U.S. Pat. No. 5,503,944, the primary electricity using device was disconnected, and the flow of fuel (hydrogen) to the anode and the flow of air to the cathode were shut off. No attempt was made to purge the anode flow field of residual fuel or to purge the cathode flow field of air, such as by using an inert gas purge. To restart the cell, fuel and oxidant were flowed directly into their respective flow fields. (The foregoing procedure is hereinafter referred to as an xe2x80x9cuncontrolledxe2x80x9d start/stop cycle.) It was found that a cell stack assembly operated in this manner experienced rapid performance decay which had not previously been observed. Further, it was discovered that a large number of start/stop cycles were more detrimental to cell performance than were a large number of normal operating hours under load. It was eventually determined, through experimentation, that both the shut-down and start-up procedures were contributing to the rapid performance decay being experienced by the cell; and it was known that such rapid decay did not occur when, in accordance with prior art techniques, inert gas was used to passivate the cell at each shut down. Examination of used cells that experienced only a few dozen uncontrolled start/stop cycles showed that 25% to 50% of the high surface area carbon black cathode catalyst support was corroded away, which had not previously been reported in the prior art.
Further testing and analysis of results led to the belief that the following mechanism caused the performance decay experienced in the foregoing experiment: With reference to FIG. 2, a diagrammatic depiction of a PEM fuel cell is shown. (Note that the mechanism to be described is also applicable to cells using other electrolytes, such as phosphoric acid or potassium hydroxide with appropriate changes in ion fluxes.) In FIG. 2, M represents a proton exchange membrane (PEM) having a cathode catalyst layer C on one side and an anode catalyst layer A on the other side. The cathode air flow field carrying air to the cathode catalyst is divided into air zones 1 and 2 by a vertical dotted line that represents the location of a moving hydrogen front through the anode flow field, as further described below. The anode fuel flow field that normally carries hydrogen over the anode catalyst from an inlet I to an exit E is also divided into two zones by the same dotted line. The zone to the left of the dotted line and adjacent the inlet I is filled with hydrogen and labeled with the symbol H2. The zone to the right of the dotted line and adjacent the exit E is zone 3 and is filled with air.
Upon an uncontrolled shut-down (i.e. a shut-down without taking any special steps to limit performance decay) some of the residual hydrogen and some of the oxygen in their respective anode and cathode flow fields diffuse across the PEM (each to the opposite side of the cell) and react on the catalyst (with either oxygen or hydrogen, as the case may be) to form water. The consumption of hydrogen on the anode lowers the pressure in the anode flow field to below ambient pressure, resulting in external air being drawn into the anode flow field at exit E creating a hydrogen/air front (the dotted line in FIG. 2) that moves slowly through the anode flow field from the fuel exit E to the fuel inlet I. Eventually the anode flow field (and the cathode flow field) fills entirely with air. Upon start-up of the cell, a flow of air is directed into and through the cathode flow field and a flow of hydrogen is introduced into the anode flow field inlet I. On the anode side of the cell this results in the creation of a hydrogen/air front (which is also represented by the dotted line in FIG. 2) that moves across the anode through the anode flow field, displacing the air in front of it, which is pushed out of the cell. In either case, (i.e. upon shut-down and upon start-up) a hydrogen/air front moves through the cell. On one side of the moving front (in the zone H2 in FIG. 2) the anode is exposed substantially only to fuel (i.e. hydrogen); and in zone 1 of the cathode flow field, opposite zone H2, the cathode is exposed only to air. That region of the cell is hereinafter referred to as the H2/air region: i.e. hydrogen on the anode and air on the cathode. On the other side of the moving front the anode is exposed essentially only to air; and zone 2 of the cathode flow field, opposite zone 3, is also exposed to air. That region of the cell is hereinafter referred to as the air/air region: i.e. air on both the anode and cathode.
The presence of both hydrogen and air within the anode flow field results in a shorted cell between the portion of the anode that sees hydrogen and the portion of the anode that sees air. This results in small in-plane flow of protons (H+) within the membrane M and a more significant through-plane flow of protons across the membrane, in the direction of the arrows labeled H+, as well as an in-plane flow of electrons (exe2x88x92) on each side of the cell, as depicted by the arrows so labeled. The electrons travel through the conductive catalyst layers and other conductive cell elements that may contact the catalyst layer. On the anode side the electrons travel from the portion of the anode that sees hydrogen to the portion that sees air; and on the cathode side they travel in the opposite direction.
The flow of electrons from the portion of the anode that sees hydrogen to the portion of the anode that sees air results in a small change in the potential of the electron conductor. On the other hand, electrolytes in the membrane are relatively poor in-plane proton conductors, and the flow of protons results in a very significant drop in the electrolyte potential between zones H2 and 3.
It is estimated that the reduction in electrolyte potential between zones H2 and 3 is on the order of the typical cell open circuit voltage of about 0.9-1.0 volts. This drop in potential results in a proton flow across the PEM, M, from the cathode side, zone 2, to the anode side, zone 3, which is the reverse direction from what occurs under normal cell operating conditions. It is also estimated that the reduction in electrolyte potential in the portion of the anode that sees air (in zone 3) results in a cathode potential in zone 2 of approximately 1.5 to 1.8 volts, versus the normal cathode potential of 0.9 to 1.0 volts. (Note: These potentials are relative to the hydrogen potential at the same operating conditions.) This elevated cathode potential results in rapid corrosion of the carbon support material and the cathode catalyst, causing significant cell performance decay.
One object of the present invention is to minimize corrosion of the fuel cell catalyst and catalyst support during shut-down of the fuel cell, and to do it without purging hydrogen from the cells with inert gas upon shut-down.
In one particular embodiment of the shut-down procedure of the present invention, after the steps of disconnecting the primary load from the external circuit and shutting off the fresh fuel flow into the anode flow field, the anode exhaust is recirculated through the anode flow field in a recycle loop to continuously bring hydrogen remaining within the loop into contact with the anode catalyst of the cells to react with oxygen that diffuses across the cell from the cathode flow field to the anode flow field. The oxygen combines with the hydrogen in the presence of the anode catalyst to produce water, thereby xe2x80x9cconsumingxe2x80x9d the free hydrogen molecules. In this embodiment, air flow through the cathode flow field is preferably continued during the recirculation of the anode exhaust to increase the availability of oxygen for diffusion across the cell to react with the hydrogen on the anode catalyst. After substantially all the hydrogen has been reacted, the recirculation of the anode flow field exhaust may be stopped and the anode and cathode flow fields may be allowed to fill with air or, preferably, are both purged with air. This completes the shut-down procedure without the use of an inert gas purge and without generating a hydrogen/air front within the anode flow field. The optional air purge assures that even the smallest amounts of hydrogen are flushed from the cell, and that the anode and cathode flow field gasses remaining in the cell after shut-down are essentially identical (i.e. 100% air).
In a variation of the foregoing embodiment, rather than rely solely on oxygen diffusion across the cell to catalytically consume the recirculating hydrogen, a small controlled amount of external air is mixed with the recirculating anode flow field exhaust upstream of the point where that exhaust enters the anode flow field inlet. This speeds up the shut-down process by increasing the speed of the catalytic reaction on the anode, reducing the cathode potential and thereby reducing the rate of catalyst corrosion and rate of catalyst support corrosion during the shut-down process. Once added, the external air is circulated with the anode flow field exhaust until substantially all the remaining hydrogen has been reacted. (Hereinafter the controlled amount of external air which is added to speed up the catalytic reaction is referred to as xe2x80x9creaction airxe2x80x9d. This nomenclature is used to distinguish the reaction air from external air that may be added later in the shut-down procedure to purge the flow fields.) The hydrogen and the oxygen in the recirculating mixture of anode exhaust and air catalytically react with each other on the anode catalyst to produce water. Since, during this step of the procedure, only a mixture of air and anode exhaust enter the anode flow field, there is no distinct hydrogen/air front traversing the anode flow field; and at no time does one region of the anode see only hydrogen and the other see only air.
The oxygen in the added external air reacts on the anode catalyst with the hydrogen to quickly consume (in a matter of seconds for all systems of a practical size) substantially all of the remaining hydrogen in the recirculating gases. A typical 70 kilowatt fuel cell has a hydrogen inventory within the fuel flow fields, reactant manifolds and the recycle loop of about 12 liters, while the recycle blower is typically sized to recirculate 400 to 700 liters/minute. An air flow rate into the fuel recycle loop of approximately 140 liters/minute maintains the air concentration below the flammability limit (explained more fully, below) while providing sufficient oxygen to consume substantially all the hydrogen in about 13 seconds.
The reaction of hydrogen with the oxygen in either the external air or with the oxygen that is transported from the cathode to the anode through the membranes results in a pressure reduction in the recycle loop. It is undesirable to have the pressure in the recycle loop drop below ambient pressure since this would result in drawing an uncontrolled quantity of air into the recycle loop due to any system leaks. However, all practical size recycle blowers and recycle loop volumes, when taken together with the quantity of external air that can safely be brought into the system, will result in maintaining the pressure above the ambient pressure thus preventing any random infusion of air into the system.
Except in certain specific instances which are described later with respect to certain embodiments of the present invention, for safety reasons, the amount of air added into the recycle loop while hydrogen is present should be less than an amount that would result in a flammable mixture of hydrogen and oxygen. More than about 4% oxygen (equivalent to about 20% air), by volume, in hydrogen is considered in excess of the flammability limit; and more than about 4%, by volume, hydrogen in air is considered in excess of the flammability limit. Thus, if the recycle loop contains 100% hydrogen, the rate of air flow into the recycle loop should initially not exceed about 20% of the total recycle loop flow rate, and is preferably lower than 20% to allow a safety margin. Although not shown, a device for measuring the ratio of oxygen to hydrogen in the circulating gases may be placed in the recycle loop and used to control valves or other devices used to feed gases into the recycle loop.
It should be noted that, during the shut-down procedure of the present invention, nitrogen naturally present in the air on the cathode side of the cell diffuses across the cell into the anode flow field along with the oxygen. Also, there is nitrogen in any air added to the recirculating anode exhaust. None of that nitrogen is consumed within the cell. Therefore, in order to add air to the recirculating exhaust, the recirculating exhaust must be partially vented. After substantially all the hydrogen has been consumed, any gas mixture within the cell containing nitrogen in excess of the amount found in air may eventually be displaced by fresh air that is allowed to enter the cell after shut-down or as a result of using a final air purge, as described above. However, it is not critical to remove excess nitrogen from the cell. For the embodiments just described, in order to speed up the shut-down process, it is preferable to reduce the cathode potential prior to shutting off the flow of hydrogen fuel to the anode flow field and prior to adding any controlled amount of external air into the anode flow field. More specifically, to quickly reduce the cell voltage while still flowing fuel to the anode, but after the primary load has been disconnected, a small auxiliary resistive load is connected across the cell. The air flow to the cathode is halted during application of this auxiliary load. The application of the auxiliary load reduces the amount of oxygen in the cathode flow field through the occurrence of normal electrochemical reactions, and this reduces the cell voltage and cathode potential. The reduction in cathode potential reduces the rate of catalyst and catalyst support corrosion upon air entering the anode flow field during the remaining steps of the cell shut-down process.
The auxiliary load is connected for a period of time long enough to reduce the cell voltage to a preselected value, preferably a value of 0.20 volts or less per cell whereby substantially all the oxygen in the cathode flow field will have been consumed. After the cell voltage has been reduced to the desired value, the fuel flow to the anode flow field is shut off and a controlled amount of air may be added to the anode flow field which continues to recirculate to catalytically react hydrogen therein, all in accordance with the previously described embodiments, until substantially all the hydrogen has been removed from the anode flow field. The auxiliary load may be disconnected once the cell voltage has dropped to the desired level; but it is preferable to keep it connected until the shut-down process is completed. Thus, if the shut-down procedure is to end with an air purge of the anode and cathode flow fields (as previously mentioned), it is preferred to maintain the auxiliary load connected across the cell to limit the per cell voltage to 0.2 volt or less during the air purge. This minimizes the cathode potential and possible catalyst and catalyst support damage that could result from a) the presence of even very small amounts of hydrogen within the recycle loop, and, b) from any difference in anode potential from the anode air inlet to the anode exhaust outlet as the purge air flows through the cell.
In yet another embodiment of the shut-down procedure of the present invention, the recycle loop includes one or more burners, in series, each having a catalytic burner element therein. After the primary load has been disconnected from the external circuit and fresh fuel flow into the anode flow field has been shut off (such as in accordance with the previously described embodiments), and preferably after the cell voltage has been reduced to 0.2 volts or less (such as with the use of an auxiliary load), a small, controlled amount of external air (i.e. reaction air) is added to the recycle loop either upstream of the burners or, most preferably, directly into each burner. Hydrogen in the recirculating anode flow field exhaust catalytically reacts on the burner element with the oxygen in the air to produce water. The exhaust from the burner is continuously recirculated through the recycle loop (i.e. through the anode flow field and burners) until substantially all the hydrogen has been reacted. As discussed earlier, the amount of air added to the recirculating anode flow field exhaust is regulated to avoid having a flammable mixture of hydrogen and oxygen anywhere within the recycle loop. Preferably the shut down process should take no more than one minute, and most preferably less than ten seconds. For that reason, a plurality of burners is preferred over a single burner.
Preferably, each burner includes a diffusion burner upstream of and in series with the catalytic burner element, and preferably integrated within the same housing. The diffusion burner includes an igniter that is used to initiate the diffusion burning of the air and hydrogen entering the diffusion burner. The diffusion burning process speeds up the shut-down process by more quickly consuming the hydrogen in the recycle stream (as compared to catalytic burning alone); however, diffusion burning alone is not as effective as catalytic burning for removing the hydrogen to the levels required of the present invention. The combination of the two provide the desired speed and substantially complete removal of the hydrogen. The flammability limits, which, as discussed above, should be observed for safety within the fuel cell system, obviously do not apply to the diffusion burner; however, the flammability limits should be observed with regard to the gas composition leaving the diffusion burning zone.
The following commonly owned U.S. non-provisional patent applications, filed on Dec. 20, 2000, describe and claim inventions related to the subject matter of this application: U.S. Ser. No. 742,497 xe2x80x9cProcedure for Shutting Down a Fuel Cell System Using Air Purgexe2x80x9d, invented by Carl Reiser, Richard Sawyer and Deliang Yang; and U.S. Ser. No. 742,481 xe2x80x9cProcedure for Starting Up a Fuel Cell System Using a Fuel Purgexe2x80x9d, invented by Carl Reiser, Richard Sawyer, and Deliang Yang. The following commonly owned U.S. non-provisional patent application, filed on even date herewith and now abandoned, describe and claim inventions related to the subject matter of the present application: U.S. Ser. No. 09/769,897 xe2x80x9cProcedure for Starting Up a Fuel Cell System Having an Anode Exhaust Recycle Loopxe2x80x9d, invented by Deliang Yang, Margaret Steinbugler, Richard Sawyer, Leslie Van Dine, and Carl Reiser.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.